Can I open 2.79b files in 2.8?
Yes and no... 
Yes if you have simple blend files, I saw that 2.8 opens them pretty easily. If you re-save them using 2.8, then, yeah you cannot open them in 2.79. Blender will crash.

No, if you have Armatures blend files (I've been using a simple biped rig from the internet, much like 3DS Max has) then you will have issues. 
Also, I saw that collections tend to disappear from one save to another... (but maybe is because the Armature rig is coming from a 2.7x version...)